Re: [XEN PATCH v1] libxl/arm: provide guests with random seed

[Julien Grall <julien@xxxxxxx>]

On 26/05/2021 10:28, Sergiy Kibrik wrote:
> Hi Julien,
Hi Sergiy,

> > > diff --git a/tools/libxl/libxl_arm.c b/tools/libxl/libxl_arm.c index
> > > 34f8a29056..05c58a428c 100644
> > > --- a/tools/libxl/libxl_arm.c
> > > +++ b/tools/libxl/libxl_arm.c
> > > @@ -342,6 +342,12 @@ static int make_chosen_node(libxl__gc *gc, void
> > *fdt, bool ramdisk,
> > >            if (res) return res;
> > >        }
> > >
> > > +    uint8_t seed[128];
> > I couldn't find any documentation for the property (although, I have found
> > code in Linux). Can you explain where the 128 come from?
>   
> I didn't find documentation either, probably that part is un-documented yet.
> This is kind of tradeoff between ChaCha20 key size of 32 (which is used in 
> guest Linux CRNG), and data size that host is expected to provide w/o being 
> blocked or delayed
> (which is 256 according to getrandom() man page). In case of 128-bytes seed 
> each byte of CRNG state will be mixed 4 times using bytes from this seed.
Ok. Can the reasoning be documented in the commit message (with a short 
summary in the code)? This would be helpful if in the future one decide 
to change the size of the seed.
Cheers,

--
Julien Grall